General key issues when comparing performance between PostgreSQL and oracle

Hi all (Hopefully this is the correct mailing list for this).

I'm working on performance tuning a host of queries on PostgreSQL 9.2 
from an application, each query having its own issues and fixes, however 
from what I understand this application runs the exact same queries on 
the exact same data in half the time on oracle and SQL server.

Are there any known differences between the database systems in terms of 
query planners or general operations (sorting, unions) that are notable 
different between the systems that would make postgres slow down when 
executing the exact same queries?

It's worth noting that the queries are not that good, they have issues 
with bad sub-selects, Cartesian products, and what looks like bad query 
design in general, so the blame isn't completely with the database being 
slow, but I wonder what makes oracle preform better when given 
not-so-great queries?

I know this is rather general and high level, but any tips or experience 
anyone has would be appreciated.


Thanks,
- Brian F